1. Field of the Invention
The invention relates to a process for exposing the peripheral area of a wafer, a process which is used for removal of an unnecessary resist on a wafer in a development process, and a device for exposure. The invention relates especially to such a process in which the unnecessary resist has a step shape and where the process is a development process performed in an exposure device.
2. Description of Related Art
In the manufacture of a semiconductor device, for example, an IC, LSI or the like, the photoresist (hereafter called a resist) is applied to the surface of a semiconductor wafer, such as a silicon wafer or the like. Next, a circuit pattern is exposed and developed, and thus, a resist pattern is formed. This resist pattern is used as a mask, ion implantation, etching, lift-off or similar processing being done. To apply the resist to the semiconductor wafer (hereafter called a wafer), conventionally, a spin coat method is used to make the applied thickness of the resist film uniform. In doing so, the wafer is rotated as the resist is applied in the center position of the wafer surface. The resist is distributed over the entire surface of the wafer by centrifugal force. In this way, the resist is also applied to the peripheral area of the wafer.
The peripheral area of a wafer, however, is only rarely used as an area in which the pattern is formed. This is because the wafer is often transported and held using its peripheral area when the wafer is subjected to various treatment processes, and because distortion of the pattern often occurs in the peripheral area, and therefore, the yield is low.
In the case in which the resist is a positive resist, the peripheral area is therefore not exposed, and as a result, the resist remains in the peripheral area even after development. The resist which has remained in the peripheral area causes impurities on peripheral devices, and as a result thereof, impurities of the wafer surface and thus a reduction in yield due to loosening or the like during transportation and holding of the wafer. Where, in particular, enhancing the function of the integrated circuit and minimization thereof proceed, this is a serious problem at present.
Therefore, a proposal has been advanced for removing the unnecessary resist in the peripheral area in the development process. To do this, a process for exposing the peripheral area of a wafer is carried out in which, regardless of the process of exposing the circuit pattern in the area in which the pattern is formed, the unnecessary resist is exposed in the peripheral area. In this process of exposing the peripheral area, light supplied by optical fibers is emitted locally onto the peripheral area as the wafer to which the resist has been applied is rotated. In this way, ring-like exposure is accomplished.
On the other hand, recently, in the exposure of the circuit pattern, a checkered pattern is gradually exposed on the wafer surface to which the resist has been applied by means of a reduction projection exposure device of the stepper type. In this case, the peripheral area in which a circuit pattern which correctly corresponds to a chip, i.e., the unnecessary resist part, cannot be exposed, so that it becomes step-shaped and its size and shape also change variously during each exposure. An unnecessary resist of this type with a step shape, likewise, causes the above described decrease of yield because there are cases in which, in the process of production of the semiconductor device, loosening or the like occurs.
Removal of this unnecessary step-shaped resist is produced, for example, using the device disclosed in Japanese patent disclosure document HEI 3-242922, by local irradiation of the unnecessary resist with light delivered by optical fibers, as the light delivered by the optical fibers is moved along the step-shaped pattern of the unnecessary resist.
In the following, using the device shown in FIG. 9 for exposing the peripheral area of a wafer, the conventional process for exposing the peripheral area of a wafer is described, in which the step-shaped unnecessary resist is exposed in the peripheral area of the wafer W shown in FIGS. 10(a) and 10(b).
Ordinarily, circuit pattern CP is formed such that an orientation flat which shows the crystal orientation of wafer W and which is hereafter called "ori-fla", or a peripheral edge shape which is called a "notch", are present. Here, FIG. 10(a) shows the case of a wafer which is provided an "ori-fla" and FIG. 10b the case of a wafer provided with a "notch".
(1) First, coordinate data of an exposure area in the orthogonally intersecting X-Y coordinates, shown in FIGS. 10(a) & (b), are stored by controller 12, shown in FIG. 9. The X-direction and Y-direction of the X-Y coordinates shown here agree with the directions of movement of X-carrier 8 and Y-carrier 10, which move in orthogonally intersecting directions. The coordinate data here relate to an instant at which the center of the circular peripheral area of wafer W and the center of rotating carrier 1 agree with one another. PA0 (2) X-carrier 8 and Y-carrier 10 are driven. Means for determining the peripheral edge area of the wafer 13, which is installed in retaining arm 7, is moved as far as the peripheral edge of wafer W. PA0 (3) Rotary drive device 2 is driven. Rotating carrier 1, on which the wafer W is held in place by suction, is rotated once. During rotation of wafer W, the position of the "ori-fla" or "notch" edge part of wafer W changes. Furthermore, it also changes according to the amount of deviation of the center of wafer W from the center of rotation of rotating carrier 1 when wafer W is put in place. During rotation of the rotating carrier, therefore, the position of the nonexposure light detected by the CCD array 53 changes according to the angle of rotation of the rotating carrier 1. PA0 (4) The information on the position of the edge part of above described wafer W is computed at controller 12, by which an "ori-fla" position or a "notch" position is determined, and furthermore, the amount of deviation of the center of rotation of rotating carrier 1 from the center of wafer W is computed. For computing the amount of this deviation, for example, the process disclosed in Japanese patent disclosure document HEI 1-243492 is used. PA0 (5) On the basis of the above described computed data, rotary drive device 2 is driven and rotating carrier 1 is rotated until the "ori-fla" position becomes parallel to the X-axis of the X-Y coordinate system. In the case of the "notch", rotary drive device 2 is driven and rotating carrier 1 is rotated until a straight line between the "notch" and the center of wafer W becomes parallel to the Y-axis. PA0 (6) Furthermore, based on the amount of deviation computed to exist in step (4), the coordinate data stored in step (1) are corrected. Coordinate data (X,Y) are corrected, for example, in FIG. 12, as (X-dx, Y-dy). Furthermore, for these coordinate data, as was described above, the amount of deviation of the position of the exit part 6 from the position of the means for determining the peripheral edge area of the wafer 13 is taken into account. PA0 (7) Based on the coordinate data corrected in step (6), the X-carrier 8 and the Y-carrier 10 are subjected to drive control. Thus, the position of the light irradiated from exit part 6 is moved to a predetermined initial position. PA0 (8) Shutter drive device 43 is moved and shutter 41 is opened. Exposure light is emitted from exit part 6. PA0 (9) Based on the coordinate data corrected in step (6), X-carrier 8 and Y-carrier 10 are subjected to drive control and a step-shaped exposure area is exposed. PA0 (10) Furthermore, in the case in which the stroke lengths of the movements of X-carrier 8 and Y-carrier 10 are not very great, after exposure of a zone of the exposure area which can be exposed, rotating carrier 1 can be rotated 90.degree. each time, and after rotation, the respective exposure area can be exposed. PA0 (a) According to above described step (6) alignment unit 20 is inserted in a predetermined position in which alignment mark WAM can be observed, as is shown in FIG. 14. Because alignment marks WAM (WAM1, WAM2) are present at two locations, two alignment units 20 are also inserted. PA0 (b) Nonexposure light irradiation device 21 emits nonexposure light and alignment mark WAM on wafer W is illuminated. The image of illuminated alignment mark WAM is determined via mirror 23, half mirror 22 and projection lenses 24, 25 by means of optical sensor 26 and is picked up by monitor 27. PA0 (c) Image data of two alignment marks WAM (WAM1, WAM2) are computed, and angle .theta. is computed as the amount of deviation. PA0 (d) Rotating carrier 1 continues to be rotated by computed angle .theta., so that circuit pattern CP (i.e., the exposure area) is in a predetermined positional relationship relative to the X-Y coordinates. PA0 (e) Based on the coordinate data corrected in step (9), the position is computed in which the respective alignment mark WAM is to be actually picked up on monitor 27. PA0 (f) The difference between the position in which alignment mark WAM is actually to be picked up on the monitor 27, and the position in which it is actually picked up is determined and computed. The coordinate data corrected in step (6) are further corrected. PA0 (g) After the above described correction, the emission of nonexposure light from nonexposure light irradiation device 21 is stopped. Alignment unit 20 is removed from one predetermined position to a removal position. PA0 (1) When each correction of the amount of deviation of circuit pattern CP is performed, alignment units 20 are inserted into the predetermined positions and are removed to the removal positions. If, however, there are errors in the positions in which the insertions into the predetermined positions are performed (positions A and B in FIG. 15), the image data of two alignment marks WAM (WAM (A) and WAM (B)) change, as shown in FIG. 15. PA0 (2) If the position accuracy of two alignment units 20 is not high, therefore, a deviation of the relative position relationship with respect to the X-Y coordinate system, which is the motion coordinate system of the X-carrier 8 and Y-carrier 10, occurs. As a result, errors occur in the correction of the coordinate data as rotating carrier 1 continues to rotate by angle .theta. in above described steps (e) and (f). PA0 (3) Therefore, devices for positioning of alignment units 20 with high accuracy are necessary; this causes increased costs. PA0 (4) Furthermore, the disadvantage of enlarging the device arise since several alignment units are necessary. PA0 (1) Due to the wafer observation means for observing the errors in the position in which in the preceding process the circuit pattern is formed, an observation device in addition is unnecessary. PA0 (2) The exit end of the optical fibers (light guide fibers) and the above described observation means are fixed integrally such that they are in a predetermined positional relationship to one another. They can, furthermore, be moved in orthogonal directions. The movement coordinate systems of the exit end and of the observation means therefore agree fully with one another. Accordingly, correction of the errors of the position in which the circuit pattern is formed can always be achieved with high accuracy, and therefore, a positioning device with high accuracy for the observation means, which is necessary if the above described coordinate systems are independent, is no longer necessary. PA0 (3) Furthermore, the measure by which an observation means is subjected to drive control, by which the predetermined two points of the pattern on the above described wafer or the two alignment marks which have a predetermined relationship to the above described pattern and which are positioned separately from the above described pattern are determined and stored separately, making several alignment units unnecessary. Therefore, a small device for exposing the peripheral area of a wafer with low costs can be devised. PA0 (1) An additional observation device is unnecessary due to the wafer observation means for observing the errors in the position in which in the preceding process the circuit pattern is formed. PA0 (2) The exit end of the optical fibers and the above described observation means are attached integrally such that they are in a predetermined positional relationship to one another. Furthermore, the swivelling rotating carrier on which the wafer is placed can be moved in orthogonal directions. The correlation between the position of the exit end and the observation means which are attached integrally to one another and the position of the rotating carrier on which the wafer is placed therefore remains the same. PA0 (3) Furthermore, the measure by which the rotating carrier is subjected to drive control, by which the predetermined two points of the pattern on the above described wafer or the two alignment marks which have a predetermined relationship to the above described pattern, and which are positioned separately from the above described pattern are determined and stored separately, makes several alignment units unnecessary. Therefore, a small device for exposing the peripheral area of a wafer with low costs can be devised.
Furthermore, a correlation is established between these coordinate data and the information on the positions of X-carrier 8 and Y-carrier 10, which are determined by means for determining the X-carrier position 9 and by means for determining the Y-carrier position 11. PA1 Furthermore, for these coordinate data, a positional relationship is also considered between an exit part 6 of optical fibers 5 which deliver exposure light from a light source 4 for purposes of exposure, and a means for determining the peripheral edge area of the wafer 13 which is described below. The exposure light source 4 consists of a discharge lamp, for example, which emits light which contains, ultraviolet rays as the exposure light, by which the resist is exposed to the action of exposure light, and of a focussing mirror for focussing the emitted ultraviolet rays and the like. PA1 The position information of X-carrier 8 and Y-carrier 10 is determined by a means for determining the X-carrier position 9 and by a means for determining the Y-carrier position 11 and is sent to controller 12. This means that drive control of the X-carrier 8 and the Y-carrier 10 is a loop control. PA1 Means for determining the area of the peripheral edge of the wafer 13 consists, for example, of a light emitting diode 51 for emitting nonexposure light by which the resist is not exposed to the action of exposure light, a lens 52 for converting the nonexposure light into parallel light, and a CCD array 53. The nonexposure light, as parallel light which has passed through lens 52, is partially shielded by wafer W and passes through the outside of an edge part of the wafer W, as is illustrated in FIG. 11. This transmitted light is detected by CCD array 53. PA1 This means that the angle of rotation of rotating carrier 1 is determined by an angle of rotation reading device 3, and for each angle of rotation, the information, which pixel under each pixel of CCD-array 53 has been detected by the nonexposure light, is ascertained. In this way, position information about the edge part of wafer W can be obtained, the angle of rotation of the rotating carrier 1 being called the parameter. PA1 Furthermore, by using the process disclosed in Japanese patent disclosure document HEI 5-3153, it can be confirmed whether the above described positioning has been done correctly. This means that, in the case of "ori-fla," the means for determining the peripheral edge area of the wafer 13 is moved along the "ori-fla" in the direction of the X-axis, the "ori-fla" position is determined, and the angle between the "ori-fla" and X-axis is ascertained. If necessary fine adjustment is performed. PA1 The image of alignment mark WAM actually picked up on monitor 27, in this process, is picked up due to the influence of further rotation of rotating carrier 1 by angle .theta. in a position which deviates from the above described position in which picking-up of the alignment mark is to be achieved. PA1 Two alignment units 20 are, however, each inserted and removed independently of one another. The resulting position data have no correlation with the position data of exit end 6 (position data of X-carrier 8 and Y-carrier 10: X-Y coordinate system). PA1 As a result, correction of the errors of the position in which the circuit pattern is formed can always be performed with high accuracy, and therefore, there is no longer any need for a highly accurate positioning device for the observation means, which is necessary if the above described coordinate systems are independent.
As was described above, in the case of exposure of the step-shaped unnecessary resist, the exposure area, which was ascertained by assuming the "ori-fla" position (or "notch" position), is stored. Furthermore, here, the position of the light irradiating from the exit part 6 in the X-Y directions, which orthogonally intersect each other, was controlled accordingly and thus exposure was performed. In doing so, it was assumed that the exposure area of the unnecessary resist is positioned with reference to the "ori-fla" position (or "notch" position) in a predetermined direction.
This means that, in the case in which the "ori-fla" was assumed, it is assumed that sides L1, L3 of the inside of the unnecessary resist area are parallel to the X-axis, which agrees with the "ori-fla" direction, and that the sides L2, L4 are parallel to the Y-axis, which orthogonally intersects the X-axis, as is illustrated in FIG. 10(a).
In the case in which the "notch" was assumed, it is assumed that sides L6, L8 of the inside of the unnecessary resist area are parallel to the Y-axis, which agrees with the direction which occurs between the corner point of the "notch" and the center of the wafer, and that sides L5, L7 are parallel to the X-axis, which orthogonally intersects the Y-axis, as is illustrated in FIG. 10(b).
However, there are cases in which, in a process of exposure and formation of a circuit pattern which is performed before this process of exposing the peripheral area of the wafer, the position of the circuit pattern CP, i.e., the exposure area of the unnecessary resist, is not positioned in a predetermined direction with respect to the "ori-fla" position or the "notch" position as a result of errors in positioning or the like, as is shown, for example, in FIG. 13. In FIG. 13, examples are shown in which a circuit pattern CP is at an angle .theta. with respect to the X-axis. As was described above, therefore, exposure of a predetermined area becomes impossible, even if the deviation of the position of the center of rotation of the rotating carrier 1 from the position of the center of wafer W is corrected.
Therefore, after exposure and formation of the circuit pattern, the circuit pattern on wafer W must be observed separately before exposure of the peripheral area of the wafer is performed, angle .theta. shown, for example, in FIG. 13, must be determined in above described step (5), rotary drive device 2 must be driven until the "ori-fla" position or the "notch" position becomes parallel to the X-axis of the X-Y coordinate system, and rotating carrier 1 must be corrected by angle .theta. during rotation.
This deviation of the exposure area changes in the process of exposure and formation of the circuit pattern according to each wafer or each production lot. Therefore, an observation must be taken each time and an observation device for this purpose must be positioned in addition.
Furthermore, with respect to the deviation of the exposure area of the unnecessary resist which is formed when rotating carrier 1 is rotated by angle .theta., a correction must be performed, by which a complex computation becomes necessary.
In order to eliminate the above described disadvantages of the prior art, at two locations on wafer W, the present inventors have positioned alignment marks WAM (WAM1, WAM2) in a predetermined positional relationship with respect to circuit pattern CP, as is illustrated in FIG. 2. The inventors have found a new process in which, by observation of these alignment marks WAM and based on the observation data thereof, the above described amount of deviation is automatically corrected.
FIG. 14 shows an arrangement of an alignment unit for observing alignment mark WAM. Alignment unit 20 comprises a nonexposure light irradiation device 21, half mirror 22, mirror 23, projection lenses 24, 25, and an optical sensor 26 which is formed by a CCD camera or the like. Reference number 27 designates a monitor which picks up signals from optical sensor 26 and sends the image data of alignment mark WAM (position data of the coordinates on the monitor) to controller 12.
Correction of the amount of deviation is performed as follows:
Then the steps beginning with (7) are followed.
In the above described correction process, using the alignment marks which the inventors found, the means for observing wafer W is positioned integrally with the device for exposing the peripheral area of the wafer. Therefore, it is no longer necessary to position an observation device in addition. Furthermore, a correction can be performed relatively easily with respect to the deviation of the exposure area of the unnecessary resist which arises when rotating carrier 1 is rotated by angle .theta..
However, in this case, the following disadvantages arise: